Previous work from this laboratory has documented deficiencies of the cell-mediated immune response in human and experimental murine leprosy. Since these granulomatous infections damage thymus-dependent lymphocyte physiology and distribution kinetics. Work in progress has demonstrated a perturbation of lymphocyte recirculation in M. lepraemurium infected rats after intravenous infusion of syngeneic, radioactively labeled thoracic duct lymphocytes from normal animals. The mechanism of this perturbation will be explored. In separate experiments, distribution patterns of theta- antigen bearing thymus-dependent lymphocytes within lymphoid tissues of infected and normal mice will be compared. Also, quantitation of theta- bearing lymphocyte populations in spleen, lymph node and thoracic duct lymph of normal and infected animals will be performed. The peripheral blood lymphocytes of humans with leprosy will be examined by a "sandwich" radio-immune labeling technique to determine if there is an abnormality in the proportion of thymus-dependent and Bursa- equivalent lymphocytes present. Pending results of the above studies, blood lymphocytes from normal donor volunteers will be labeled in vitro with Na sub 2 51CrO sub 4. Labeled cells then will be either auto-transfused or transfused to leprosy patient volunteers who possess identical ABO and HLA-histocompatibility phenotypes. By this technique, it may be possible to study the kinetics of lymphocyte survival in the circulation of lepromatous leprosy cases as compared with normals.